Automotive design and manufacture present a unique set of problems in the selection of materials and processes used in the manufacture of automobile bodies. Recent trends in the selection of construction materials for automobile bodies and their components include the use of plastic materials for many components while the main body panels continue to be made principally from sheet metal. The use of combinations of materials for the construction of the different components of an automobile presents unique problems in the matching of the paint finishes of the different components. Matching metallic paint finishes has been especially problematic.
For example, for automobile body components made of sheet metal, painting processes have generally included either spray painting the body panel or sometimes dipping the body panel or even a partially assembled car body. In either case, the painted finish is then baked at high temperature to harden the painted surfaces thoroughly. Most such paint systems use acrylic or urethane enamels that chemically crosslink during the baking step to form a hard, glossy, durable paint coat.
In developing such baked-on painting processes, it has been determined that various visual effects can be created by varying the orientation of the metallic flakes used in a metallic paint finish. Sometimes the metallic flakes are oriented so as to be essentially parallel to the outer surface of the paint layer. These metallic paint finishes have what is known as high "flop" and produce bright, often desirable visual effects. Other times, a more random orientation of the metallic flakes is sought in order to achieve a different, more appealing metallic appearance. Such a random orientation of the metallic flakes in a baked-on and crosslinked acrylic or urethane enamel paint finish has been achieved by traditional spray painting techniques. Sometimes the random flake orientation has been enhanced by the addition of microspheres to the paint, as disclosed by Backhouse in U.S. Pat. No. 4,403,003.
In recent years the trend toward use of more plastic automobile body components has led to the development of new painting techniques. An initial problem to be overcome was that the plastic auto body panels and parts cannot be subjected to high temperatures for solvent evaporation and curing as is done in traditional sheet metal painting processes.
In order to overcome this problem, processes for making thermoformable paint films were developed. According to these processes, a paint coat comprising a polymeric material is cast in thin film form onto a flexible heat-resistant temporary casting sheet. The paint coat is then dried by passing the sheet through an oven, followed by removing the paint coat from the casting sheet and transferring the paint coat to a thermoformable backing sheet. The paint coat on the thermoformable sheet retains the durability, gloss and many other appearance properties necessary for exterior automotive applications. The thermoformable paint sheets then can be thermoformed into the complex three-dimensional shapes of the plastic automobile parts or panels without detrimental effect on the appearance properties of the paint finish. The thermoformed sheet then can be molded to a plastic substrate body panel or part in an injection mold. This is known as an "insert mold" process. Alternatively, the thermoformable sheet can be shaped in the mold by the molding material itself in an "in mold" process.
Such processes have been successful in achieving paint coats with high gloss levels, high distinctness-of-image (DOI), excellent durability and other desirable properties. Generally, the paint coats comprise a clear coat and a separate pigmented color coat. The pigmented color coat can also include reflective flakes to give the paint a desirable metallic appearance. The clear coat and color coat may be formed as separate thin film coatings which are dried and bonded to one another. While the use of a clear coat is optional, it is generally desired because it contributes to higher gloss for the finished paint film as well as better durability and weatherability. The multilayer paint coat can be formed by the sequential application of first a clear coat and then a color coat to the temporary casting sheet. The paint coats are applied in this order because the smooth casting sheet surface imparts a high gloss to what will ultimately be the outermost surface of the paint film once removed from the casting sheet and applied to a substrate.
In one process disclosed in U.S. Pat. No. 4,931,324 to Ellison et al., a clear coat is applied by reverse roll coating and a pigmented color coat containing flakes is applied by conventional spray painting techniques. The disclosed spray paint techniques and the polymeric materials used in the color coat produce what is described as an essentially parallel orientation of the reflective flakes. U.S. Pat. No. 4,769,100 to Short discloses another process involving a decorative sheet applied to a contoured substrate by a shrink wrap process, in which metallic automotive paint films are applied to a stretchable carrier by spray painting techniques. The flakes are said to be essentially parallel and remain essentially parallel after vacuum forming to conform to the substrate surface.
Processes using paint transfer films for applying automotive paints to molded plastic panels and parts have been carried out successfully using thermoplastic paint films made from an alloy of fluoropolymer and acrylic resins. Such processes have included a first step in which a clear coat is made by preparing a solution of a fluorinated polymer such as polyvinylidene fluoride (PVDF), an acrylic resin such as polymethyl methacrylate, and a solvent. Alternatively, the PVDF may be present as a dispersion in a solution of acrylic resin and the solvent. The polymeric material is applied to a casting sheet and dried by passing it through an oven. A color coat is then applied over the clear coat. The color coat contains basically the same polymeric materials as the clear coat except that it also includes the pigment, and reflective flakes also can be added to achieve a metallic paint finish. The color coat is then dried and optional size coats and/or backing sheets can be applied to the color coat by the methods described previously.
It is important to recognize that these thermoplastic paint films, though dried and hardened by a heating process, are very different from the hard, baked-on acrylic and urethane enamels that are applied to preformed components by standard sheet metal spray painting techniques. Since thermoplastic paint films do not crosslink, they are not hard and brittle like the thermoset films, but rather are capable of elongating during thermoforming without cracking, hazing, or otherwise resulting in unacceptable paint finishes.
In making the thermoformable paint films that are preferred for dry paint transfer films used in coating plastic automobile components, the traditional casting of solvent-based liquid paint coats, such as by roll coating processes, tends to produce a parallel orientation of the reflective flakes. Such coating techniques do not achieve the more random orientation of reflective flakes that has become a preferred metallic appearance and which is characteristic of the spray painted hard, durable thermoset paint coats used in the prior art. Spray coating of thermoformable paint films on a carrier sheet is said to produce an essentially parallel orientation of the flakes, as disclosed in Ellison et al. U.S. Pat. Nos. 4,931,324 and Short 4,769,100, mentioned previously. U.S. Pat. No. 5,132,148 to Reafler also discloses a process for making a decorative transfer film for exterior automobile body panels. In this process a solvent-based polymeric color coat with dispersed reflective flakes is extruded onto a stretchable carrier sheet. This process is said to align the flakes in a generally parallel orientation and produces high "flop."
Spray painting is not a desirable approach in producing decorative transfer films because it requires low viscosity, low solids paint coats with a high percentage of solvent. Elaborate and expensive environmental controls are required in order to reduce the solvent emissions during the coating process. Roll coating processes have heretofore been unsuccessful in achieving a random orientation of the reflective flakes in a metallic paint film to produce desirable metallic appearance effects. Extrusion of paint coats containing pigments is a difficult process in which to achieve uniform coloration in the finished film.
Thus, there is a need for a process for making a dry paint transfer film in which a pigmented metallic automotive paint coat achieves a sufficiently random orientation of the reflective flakes to closely match the metallic appearance of conventional spray painted metallic paint coats. Such a process using roll coating techniques is desirable to avoid the drawbacks of spray painting and extrusion of pigmented films containing flakes.